1. Research Associate, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews, Urbana, IL 61801 (corresponding author); present address: Dept. of Water Resources and Environmental Engineering, Tamkang Univ., Taipei County, Taiwan.
2. Chester and Helen Siess Professor, and Director, Ven Te Chow Hydrosystems Lab., Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews, Urbana, IL 61801.